1. Field of the Invention
This invention relates to a surface treatment process for smoothing surfaces of molds and cores of a mold (hereinafter collectively referred to as a mold) composed mainly of sand and a mold produced by this method.
2. Description of the Background
Conventionally, a mold uses relatively coarse sand since it requires strength and permeability. Accordingly, metal penetration often occurs, that is, molten metal penetrates into the mold walls, and the casting is not easily removed off the mold.
In addition, some castings may be used as it is without finishing. Namely, the casting with as-cast surfaces may sometimes be used as a product. Roughness of as-cast surfaces may even determine the value of product. However, castings made by using the mold made of coarse sand hardly had fine cast surfaces. In general, a rough cast surface usually does not give good accuracy and appearance.
Thus, it has been employed to coat the surfaces of mold with fine sand. As done in the coating, an alcohol, water and a binder if necessary, are added to the fine sand to make slurry, and the slurry is applied on the surfaces by brushing, spraying, and dipping.
However, the coating process requires a drying process. Cracks and blisters occur during the drying, and the coating material tends to peel off the surfaces. Further, when the coating material is likely to adsorb water, it takes a lot of time to dry the coating material. Accordingly, the number of drying processes and the manufacturing cost increase. Furthermore, if the coating material is not dried sufficiently, water adsorbed in the coating material vaporizes during casting, and the generated gas may cause defects.
The problems mentioned above will be described briefly with reference to FIGS. 9 and 10. When a mold surface 31 is rather coarse as shown in FIG. 9, molten metal 5 penetrates into depressions between casting sand particles 30 of the mold surfaces 31 to cause metal penetration 51 and makes the cast surface rough. The mold surface 31 is coated with a coating material 6 in a wet process as shown in FIG. 10 to make the cast surface smooth. But it requires a drying process to dry the coating material 6, and the coating material 6 tends to peel off easily. In addition, the uneven thickness of coating material 6 adversely affects the accuracy of the mold, and a running 61 of coating material 6 due to the surface tension occurs at the edge of mold. The running 61 results in uneven thickness of coating material 6, and adversely affects the dimensional accuracy of product.
Further, the shell molding process has been widely employed for manufacturing a mold. This is because a casting with high dimensional accuracy can be obtained when casting is done with a mold manufactured by the shell molding process. The shell molding process utilizes the thermosetting property of a synthetic resin like phenolic resin. For casting sand, resin-coated sand is usually used in the shell molding process. The resin-coated sand is silica sand particles coated with a thermosetting resin like a phenolic resin. A cold setting resin like cold box is sometimes used in the shell molding process.
When casting molten aluminum, magnesium or an alloy thereof in a mold made of casting said coated with the cold setting resin or the thermosetting resin mentioned above (hereinafter referred to as resin) at a relatively low casting temperature, the resin decomposes insufficiently and gum generates. The gum adheres in gas vents, and blocks to let out the generated gas. The generated gas flows back to cavities of mold, and results in defective castings. To prevent this problem, the gum adhered in the gas vents should be cleaned so frequently that the cost for maintaining the mold increases.
In an attempt to resolve the above defect, Japanese Unexamined Patent Publication (KOKAI) No. 54244/1985 proposes a process for improving a mold surface defining shape of castings by introducing a mold in a fluidized bed of a pulverous refractory powder, vacuuming the mold in the fluidized bed and thereby filling up depressions of the mold with the treating powder. In this process, an evacuated space has to be provided at the opposite side from the mold surface defining the shape of castings. Therefore, this process is hardly employable for a mold having a complicated shape, because the power of sucking the pulverous refractory powder is weak at the complicated shape portion of the mold.